1. Field of the Invention
This invention related to passive acoustic reflectors and markers for underwater use.
2. Discussion of Prior Art
Passive acoustic reflectors are known, for example, from UK Patent GB2437016A (THE SECRETARY OF SATE FOR DEFENCE) Oct. 10, 2007 (incorporated herein by reference). In UK Patent 2437016 a passive acoustic reflector has a wall arranged to surround a core, said shell being capable of transmitting acoustic waves incident on the shell into the core to be focused and reflected from an area of the shell located opposite to the area of incidence so as to provide a reflected acoustic signal output from the reflector, characterised in that the core is in the form of a sphere or right cylinder and is formed of one or more concentric layers of a material having a wave speed of from 840 to 1500 ms−1 and that the shell is dimensioned relative to the core such that a portion of the acoustic waves incident on the shell are coupled into the shell wall and guided therein around the circumference of the shell and then re-radiated to combine constructively with the said reflected acoustic signal output so as to provide an enhanced reflected acoustic signal output.
More recently other potential applications for acoustic reflectors have emerged to mark underground, gas pipes and the like, which unlike other underground objects, such as electricity cables, are very difficult to detect using conventional electro-magnetically based detection systems.
Known reflectors including those seen in WO2009/122184A (published after the earliest priority date of the present application) comprise hemispherical shells. When the core is solid, such as an elastomer, the elastomer is cast into the two halves of the shell. This is labour intensive in manufacture because of the need carefully to check and top up the core castings after initial solidification and to ensure a flat level surface on the tops of the two halves of the core. The performance of such is not always predictable and furthermore when made according to the construction described will distort and/or fail when immersed in water of any depth, or when raised. This is because:                a. The separate halves of the core are joined together, this causes a discontinuity in acoustic paths in the shell, with the risk also of small gaps between the parts of the core, with consequential structural weakness and huge variations of performance between one reflector and another;        b. The shells and ores are glued together, and are inherently of weak construction for deployment in water of any depth.        